If AD5625R already tied the ref pin to external voltage, can software still program it to activate internal reference or it will be overrode by the external Ref voltage or it will conflict at ref pin?

Answer:

The Vrefin/Vrefout pin serves as:

1. Reference input pin when external reference is selected and
2. Reference output pin when internal reference is selected.

Initially, the internal reference is disabled at power-up so the voltage at Vrefin pin will be used. Then, using the reference setup command, you can choose whether the internal reference will be on or off (default). If you're planning to use the internal reference, the external supply to the Vrefin/out pin should ideally be disconnected. AD5625 is available with and without internal voltage reference option.